The Exploration of the Moon

The successful mission of Apollo 11 opens an epoch of planned lunar exploration. What questions should this exploration seek to answer, and what areas of the moon should be visited to best confront them?

Editor's note: This article originally appeared in the October 1969 issue of Scientific American magazine. We are posting it to commemorate the 40th anniversary of Apollo 11's moon landing.

The success of the Apollo 11 mission in putting men on the moon and bringing them back safely with samples of lunar material marks the beginning of what promises to be a period of fruitful exploration of the moon by men and machines. The objective will be to answer a large number of questions about the origin and evolution of the moon, its geology, its chemical and physical structure and what light its history can shed on the history of other bodies in the solar system. Our purpose in this article is to discuss the major questions the coming manned expeditions to the moon will be taking up and to describe the techniques likely to be employed on such missions.

In an astronomical sense the moon is usually considered to be a satellite of the earth. From the viewpoint of planetary processes, however, the moon can be regarded as the smallest of the "terrestrial" planets (the others being the earth, Mars, Venus and Mercury). Because its distance from the sun is about equal to that of the earth, the moon is subject to external influences similar to those affecting the earth. The moon's smaller size, however, implies a history quite different from the earth's.

Most planet-wide processes result from internal sources of energy and the means of its dissipation. The amount of internal energy and the means of dissipation are dependent on the size of the planet. On the earth the dissipation of energy has been accompanied by the transport of volcanic fluids from the interior of the earth to the surface, by the long-term development of a light crust and a dense core and by large-scale movements of the earth's crust, mantle and core. These processes, together with erosion and the chemical interaction of materials on the surface with the atmosphere and the hydrosphere, continually destroy the earth's surface features. For example, even the largest volcanoes are leveled by erosion within a few million years after volcanic activity ceases. It is extremely unlikely that any of the earth's original surface features still exist unchanged. No examples are known, and almost certainly none will be found.

It was once thought that many of the surface features of the moon date back to the moon's formation. The detailed views of the lunar surface provided by the photographs from the Lunar Orbiter space vehicles have somewhat diminished this possibility, because they indicate that erosion and other processes of change do take place on the moon. Preliminary analysis of the samples returned by Apollo 11 nonetheless indicates that the material is very old, perhaps three billion years old. The highlands may be even older. The possibility that some of the material lying on the lunar surface is chemically unchanged since the formation of the planet remains high.
The Major Questions
Fundamental scientific questions about the moon are often stated in terms of terrestrial characteristics, which are of course more familiar. In inquiring about the gross chemical and physical structure of the moon, for instance, one wonders if the moon is chemically and mineralogically differentiated as the earth is. The processes such as volcanism have occurred on the moon, what has been their history over long periods of time? Did the moon ever have an atmosphere? Have protobiological materials ever existed or evolved on the moon?

Answers to such questions call for the recovery and analysis of samples of lunar materials from a variety of regions on the moon. Most of the analysis will have to be done on the earth. Determining the age of a sample of lunar material or making a chemical and mineralogical analysis of it requires instruments that cannot be deployed on the lunar surface within the next few years, particularly with little or no prior knowledge of the character of the materials to be analyzed.

The study of returned lunar materials will in fact provide one of the most intriguing challenges ever faced by natural scientists. How much of the moon's history and how many of the lunar surface processes can be understood from a few isolated samples of lunar material, aided by the fairly detailed knowledge of the surface morphology obtained from photographs? The possibilities are considerable, because the lunar surface is not subject to many of the chemical processes that occur on the earth's surface, such as the changes accompanying erosion and sedimentation. Furthermore, the distribution of material over the surface of the moon by the impact of meteorites suggests that a substantial amount of material in any given place may have come from great distances without significant changes in chemical composition.

Efforts to trace the evolution of the moon, to understand its gross internal structure and to explain the characteristics of its major morphological features will require knowledge of the kind and amount of internal energy released by moonquakes, heat flow at the surface and volcanism. The occurrence of moonquakes would reveal something of the distribution of stress with depth. The seismic waves arising from moon quakes would provide a powerful tool for deducing the distribution of basic physical properties with depth. Measurement of heat flow at the surface, combined with estimates of the distribution of radioactive elements in the lunar rocks, would make possible a determination of whether or not internal energy is in fact the cause of volcanism on the moon. Data for attacking these problems will be needed from a number of widely distributed points on the moon.

The Problem of the Mascons
The space vehicles employed in the Lunar Orbiter missions not only made excellent photographs of the lunar surface but also yielded a startling discovery having to do with the gravitational field of the moon. If the moon were a symmetrical spheroid, internally as well as externally, a satellite would move around it in a well-defined elliptical orbit at a smoothly varying speed. In actuality the moon, like the earth, is not quite a symmetrical spheroid, which introduces perturbations in satellite orbits. Over and above these perturbations, however, there are others introduced by lateral variations in the moon's density. As the Lunar Orbiter vehicles were tracked in their orbits it was noted that they gained speed whenever they passed over one of the moon's ringed maria, or dark circular "seas". Analysis of these motions by Paul M. Muller and William L. Sjogren of the Jet Propulsion Laboratory led to the finding that over the major circular maria (Imbrium, Serenitatis, Crisium, Humorum and Nectaris) there is a substantial excess of gravity.

What is the cause of these gravitational variations? The large positive anomalies associated with the maria imply concentrations of mass, now abbreviated as "mascons". An example of the concentration involved is provided by the estimate that the gravitational anomaly over MareImbrium is equivalent to one produced by a sphere of nickel–iron 70 kilometers in diameter centered at a depth of 50 kilometers.

The discovery of lunar mascons has given rise to much speculation and debate about their origin. It has also revived interest in exploring the lunar maria, which many investigators had dismissed as unlikely to be as rewarding scientifically as other areas of the moon. Do the mascons represent remnants of giant iron asteroids that struck the moon and subsequently were buried and fragmented, or were they formed by some other mechanism?

Most students of the moon favor the latter possibility. The debate centers on what the mechanism might have been. Several mechanisms have been proposed: the filling of a low-density, fragmented lunar crust with lava; a flow of lava into an impact crater; the upwelling of denser material from the lunar depths into giant impact basins; even the deposition of sediment in the maria by flowing water that later dried up. The last hypothesis carries the intriguing implication that water not only existed on the moon at one time but also played an important role in lunar history. In any case, the analysis of samples from the moon takes on added significance as a result of the mascon phenomenon. An exciting result from the preliminary measurements of Apollo 11 samples is that their density of 3.2 to 3.4 grams per cubic centimeter, which would be high for terrestrial rocks, may be related to the existence of the mascons.

Sites for Exploration
The present plan of the National Aeronautics and Space Administration is to make nine more manned explorations of the moon over the next three or four years. The sites for the first few will probably be determined on the basis of constraints similar to those that were in effect for the Apollo 11 mission, namely that a landing place must be on the side of the moon facing the earth, so that constant radio communication can be maintained between the earth and the landing party; that the site be in a region free of obstacles, and that it be accessible from a free-return orbit, meaning an orbit that will enable the astronauts to return to the earth with a minimum of power if the main engine in the command module should fail. These constraints restrict the next few landings to mare sites near the lunar equator.

Later it should be possible to venture farther afield and to land at or near other sites of particular scientific interest. A number of places are under discussion as possibilities for these landings, Instead of describing them all, we shall focus on four candidate sites and a long-traverse area that we believe offer significant clues for deciphering lunar history. No particular significance should be attached to the order in which we discuss the sites.

The first site is the small, extremely fresh crater Censorinus, A landing here could be expected to achieve three objectives: to establish the age of what is clearly a very young feature on the lunar surface, to investigate and characterize an unquestioned impact feature and to obtain samples of material from a region in the highlands, An alternative site, which would offer similar possibilities, is the crater Mosting C.

The second site represents the much more ambitious goal of exploring one of the major craters. Such a crater is Copernicus, which is about 70 kilometers in diameter and has prominent central peaks within it. The ejecta from this relatively young crater cover more than a tenth of the front face of the moon. The relief within the crater is more than 15,000 feet [4,572 meters], making it comparable to the most mountainous areas on the earth. An alternative site, with quite similar characteristics, is the crater Tycho. These large craters are of interest not only because they represent major events in the history of the moon but also because, by analogy with much smaller terrestrial craters, they should expose material from a range of depths up to 10 kilometers, and perhaps even more. It has been suggested that the central peaks in these craters may consist of material now at the surface that has come from depths of 10 to 15 kilometers or more. Thus, even though the material in a crater may be jumbled, broken and deformed by shock processes, it should provide a diverse sample of the outer few kilometers of the moon and a basis for interpreting its history.

Third, we point to the extremely interesting Marius Hills region. It is one of several areas where constructional features such as domes and built-up cones are more numerous than craters of a comparable size. The region is also associated with one of the longest lunar ridge systems, which crosses a large expanse of Oceanus Procellarum on the western half of the moon. The tectonic setting of the region is similar to that of terrestrial volcanic fields such as Iceland and the Azores. The setting and structure of the Marius Hills region suggest that it is an area of volcanic activity where igneous material has been added to the surface through vents.

The origin and age of the seemingly volcanic features in the Marius Hills region are of considerable importance in understanding the evolution of the lunar surface. Terrestrial volcanic features are built up in very short times compared with the entire history of the earth. Even an extensive region such as the volcanic chain constituting the Hawaiian Islands represents a period of less than 70 million years. The absolute age and the length of time involved in building up the Marius Hills domes will be of great interest in the characterization of lunar volcanism.

The Marius Hills region is far too extensive to be covered in a single manned expedition to the moon. Fortunately a number of characteristic features of smaller scale can be visited in several areas that are no more than 70 kilometers in diameter. A mission to such an area would be able to sample and study a number of small domes 50 to 100 meters in elevation with convex slopes; steep-sided domes with rough, intricate surfaces; steeply convex or bulbous domes that are smooth and generally symmetrical; steep-sided cones with linear depressions at the summit; narrow, steep-sided ridges, and a variety of impact features.

The fourth candidate site is the region of the Apennine Mountains, which roughly form the southeastern boundary of MareImbrium and also the northwestern leg of a triangular highland area bounded by MareImbrium, the southwestern boundary of Mare Serenitatis and the northern part of Sinus Aestuum. The Apennines are among the most impressive of the lunar mountain ranges. The Apennine front rises 4,800 meters above the adjacent mare level to the west.

What can be learned about the moon by visiting this area? The Apennine front is a major physical feature of the moon, exposing an extensive vertical section several thousand meters thick for sampling and examination. Here is an opportunity to assess what may be a long period of lunar history. Are the rocks in form or physically and chemically heterogeneous? How old are they? Are they stratified? Answers to such questions could have a profound effect on our understanding of lunar history.

Two landing sites have been proposed near the Apennine front that are within five kilometers of important lunar features. One such feature is the rille, or canyonlike configuration, known as Rima Hadley. Is it a surface-flow channel or a collapsed lava tube? If it was formed by water, as has been speculated, where did the water come from and what prevented its immediate evaporation?

The Significance of Rima Hadley
Close examination of the Lunar Orbiter photographs of this rille reveals that fresh exposures of rock are visible along its walls and that blocks have fallen down the walls to the floor of the rille. Rima Hadley cuts into the Boor of a mare and thereby yields a depth and perhaps a cross section of the history of a major lunar feature. Hence it might provide answers to such questions as whether the maria are bedded deposits of lava or ash flows, sedimentary deposits that contain a sequential history of formation or simply an agglomeration of cold particulate matter accreted from space.

The location of the proposed landing site at the boundary between a highland and a mare provides the opportunity for another promising investigation. Deployment of a multiple-axis seismometer and recording of seismic waves from different directions should reveal something about any deep structural differences between the maria and the highlands. Thus, one might answer the question of whether or not the maria and highlands are analogous to the oceans and continents on the earth, which show major structural differences.

The Long Traverse
After the early fixed-station landings at a wide variety of lunar sites some form of long-range, mobile surface exploration will be necessary to overcome the limitations of men on foot. The answer lies in vehicular traverses, which would make it possible to study cross-country variations on the moon and so form the bridge between the intensive observations that can be made in the vicinity of a landing site and the extensive averaging observations that can be made from orbit. The technique that is attracting particular interest at present is called the dual-mode lunar surface roving vehicle system. The term dual mode refers to the fact that the vehicle can be used by the astronauts while they are working on the lunar surface and can be operated remotely from the earth after they depart. The present plan entails two separate lunar landings 500 kilometers apart at sites chosen to maximize the amount of information returned from the unmanned, automatic traverse.

Such an operation would proceed as follows. Near the end of surface activities in the first landing the men would start their unmanned vehicle on an automatic traverse. The vehicle, guided from the earth, would move across the moon toward a distant point that is within the second landing area. There the men participating in the second landing would meet the vehicle several months after it had started its journey. During its traverse the vehicle would collect samples of rock, transmit television pictures and conduct geophysical experiments yielding data that would be transmitted to the earth by telemetry. After the rocks had been retrieved by the astronauts at the second site the vehicle could be used by them in the exploration of that site. If the vehicle was still in satisfactory condition, they could start it on another long traverse.

A typical traverse might go from Rima Hadley into MareImbrium and thence into Mare Serenitatis. Along the way it would provide continuous profiles of the variations in gravity, magnetic and electric fields and depths of the surface layer. This particular traverse crosses one of the largest of the mascon areas and would cover enough ground to explore the phenomenon adequately with geophysical techniques.

The continuous monitoring of gravity along the traverse would provide information on the regional isostatic balance on the moon, that is, whether the higher topographic features are compensated by a deficiency of mass below them or whether they represent loads on the surface. An answer to this question would tell a great deal about the mechanism of formation of such features. Gravity information will also yield clues about the maximum depth of variations in density. If the moon has a crust analogous to the earth's, how does it vary between the lunar highlands near Rima Hadley and the center of the Imbrium basin?

The value of gravity measurements is increased if they can be combined with seismic information. Seismic measurements could be expected to resolve details of any layering in the lunar substrata. Along the traverse we have been describing a properly executed seismic experiment would quickly reveal the presence of giant iron asteroids buried in the mascons.

Improved Capabilities
Implicit in the accomplishment of the missions we have discussed is a considerable improvement of the already substantial capabilities shown in the Apollo 11 mission. We shall enumerate a number of improvements that we think can be expected by 1972, although we should point out that the estimates are somewhat uncertain. The present capability is for a mission lasting 10.8 days, with a total of 22 hours spent on the moon; by 1972 a 16-day mission with 78 hours on the moon should be possible. The payload of scientific instruments delivered to the surface should increase from 300 to 600 pounds and the amount of material of scientific interest returned to the earth from 150 to 300 pounds. Landings, which are now limited to the equatorial zone, may be possible on most of the front face of the moon, and it may also be possible to land within 0.5 kilometer of a target area instead of within 10 kilometers as now. For men outside the lunar module the walking radius on the moon should increase from 100 meters to four kilometers, and the total distance covered during a single extravehicular activity from 500 meters to several kilometers. The capacity of the life-support pack worn by the astronauts as they move about the lunar surface may be increased by as much as 50 percent from the present 4,800 Btus. It may also be that the command module will be able, while it is in orbit around the moon, to launch a subsatellite that could make additional measurements.

These new capabilities seem within reach when considered individually. It will not be possible to have them all, because a few are mutually exclusive. For example, if it were decided to land a 600-pound load of scientific instruments on the moon, it probably would not be possible to equip the astronauts to traverse as much as 10 kilometers of the lunar surface.

It also seems probable that a constant-volume suit will be available for lunar astronauts soon. With the present variable-volume suit the astronaut has to do a considerable amount of work against the suit. He also cannot bend his waist or his ankles. The constant-volume suit will require about 30 percent less work for equivalent tasks because almost no work will have to be done against the suit. It will also be flexible at the waist and the ankles. With this suit the astronaut should have considerably more mobility on the lunar surface.

Even so, several of the sites we have discussed cannot be explored adequately unless the astronauts have more mobility than walking provides. Indeed, the radius of mobility ought to be about 30 kilometers from the landing site. Obviously a vehicle will be needed. Two approaches are possible: the vehicle could crawl along the lunar surface at a few miles per hour or it could fly over the surface at low altitude. The ground vehicle has the advantage of enabling the occupants to stop and look at interesting objects, whereas a rocket-powered flying platform makes it possible to move rapidly from one point of major interest to another. A flying vehicle could also move vertically, as will be desirable at certain lunar sites. Although both flying and crawling vehicles have distinct advantages, both are expensive, and it may well be that only one capability will be developed.

Inasmuch as the most that can be expected of a landing party is the exploration of 10 to 100 square kilometers in the vicinity of the landing site, the nine additional landings now planned will cover only about one part in 10,000 of the front face of the moon. In order to obtain a more comprehensive picture of the surface, including the far side, and to look for classes of features missed at the landing sites, NASA plans to use instruments mounted in the service module for remote sensing from orbit. The sensors will be put into service starting at about the sixth landing.

Orbital Sensing Instruments
Eight types of instrument are under consideration for the remote-sensing activity. Spectrometers measuring gamma rays, x-rays and alpha particles emerging from the lunar surface would be able to detect several elements. The gamma-ray instrument could ascertain the amounts of iron, potassium, thorium and uranium in the top foot of the lunar surface. The x-ray instrument would receive radiation excited by the sun in a very thin surface layer and give information on the concentration of major elements such as silicon, magnesium and aluminum. The alpha instrument would reveal if there were any extensive leakage of radon gas from the lunar interior, such as often accompanies volcanic or hot-spring activity on the earth. An infrared radiometer would measure infrared emission from the surface and thus would be able to find hot spots and volcanic activity. A gas mass spectrometer would measure the number and type of atoms around the service module, thereby determining the density and composition of the vanishingly small amounts of gas at lunar orbital altitudes. An electromagnetic sounder would bounce pulses of radio waves (10 kilohertz to 100 megahertz) off the moon and measure how much came back, thereby finding out about subsurface layering and determining whether there is chemical differentiation or even possibly a layer of ice. A metric camera would photograph most of the moon with good geometric control in order to determine how out-of-round the moon is and whether the centers of maria are lower than the edges. A laser altimeter would bounce a light beam off the lunar surface to measure altitude accurately; such measurements, taken together with orbital data and information from the metric camera, would help to determine the moon's shape.

This kind of broad coverage would mesh well with the detailed coverage astronauts on the surface would make of small areas. Each landing would provide a standard for the orbital experiments by measuring in detail what the instruments should see from orbit. The orbiting instruments then would yield a far broader coverage of surface characteristics than could be obtained from manned landings alone.

Attainment of the goals we have described will still leave several exciting frontiers for lunar exploration. They include visits to Mare Orientale, the polar region and the far side of the moon. Such visits will require the development of a new technology.
Long-Term GoalsMare Orientale, the huge "bull's-eye" feature discovered in Lunar Orbiter photographs, is on the far western edge of the moon as viewed from the earth. It is a splendidly preserved, concentrically layered feature probably formed by the impact of a giant meteorite. The feature offers an unparalleled challenge for exploration, but it also presents large operational difficulties for a landing. The Cordillera Mountains, which ring the Orientale basin to form a circular outer scarp some 960 kilometers in diameter, are among the most massive on the moon, rising some 18,000 feet above the adjacent terrain. Perhaps this site, of all the possible ones on the moon, offers the best opportunity for studying the evolution and history of the moon.

A polar landing is a particularly fascinating prospect. Areas near the poles are in permanent shade, so that one might hope to find frozen ammonia, carbon dioxide, water and similar volatiles that otherwise would have escaped from the moon long ago.